if you dont wana use a dozen different materials, id say try getting a
pine (cheap) or hardwood (oak, maple for more durable and realistic weight diferential)

i was going to ask similar question, but after reading yours...
(its easier to answer someone elses ?'s than ones own)
(was still posted in general when i typed this)

i thought id go w/ wood, since im looking to finish with a sword i can swing about haphazardly,

after googling soul eater (not familiar w/ the title) i assume you mean the scythe w/ the red/black blade and white/silver haft

IMO, weapons should always be proportioned to the intended wielder, that said,
if your friend is only ~5'3, the over all hight should be little more than ~5'8
so that if she stood under the blade where it meets the haft, it should have a 3-4inch clearance,

the blade looks to be ~3/4 of the overall hight so youll want a plank 6in wide by 5' long
and 3in thick
(you can cut 2 blades from this) id use oak or better cause of weight distribution and integrity, for both haft and blade

joining the parts, im not sure on, one thing i am sure on is, "use powertools" it puts less stress on your parts
and remember, when using wood you gota use a "sealant" before painting
(not laquer, stain or varnish)

Disclaimer: Props are my weakest cosplay skill. But I've picked up a few things that may be relevant here.

Wood is normally a good go-to...but in this case I'd shy away from it. Wood can get really, REALLY heavy, and that blade is HUGE. If you made the blade and pole out of wood, I'd be worried about the pole snapping from the weight if it's not held close enough to the blade. Plus the weight would make it really hard to carry and pose with.

So some sort of foam would probably be better. (Gah there was one in particular but I can't remember it's name sorry!) I have a friend who coats foam in resin and it makes lightweight props that are fairly sturdy...not wood-sturdy but they hold up just fine for posing and cons. There are other things you can put over the foam to make it stronger and give it a solid look, but I don't remember what they are.

If you look in my WIP gallery, you'll see that I made a 6 ft boomerang out of foamboard. Honestly, if I were to make this prop, a double-layer of foamboard with a slate of balsa wood in between and a heavy duty wood pole is the route I'd try.